LONDON — The United States and Britain formally accused Iran on Tuesday of supplying short-range ballistic missiles to Russia to use against Ukraine, announcing new sanctions on Moscow and Tehran before a joint visit to Kyiv by their top diplomats.

Secretary of State Antony Blinken, speaking alongside British Foreign Secretary David Lammy during a visit to London, said Iran had ignored warnings that the transfer of such weapons would be a profound escalation of the conflict.

He told reporters that dozens of Russian military personnel had been trained in Iran to use the Fath-360 close-range ballistic missile system, which has a maximum range of 75 miles.